The present invention relates generally to a zoom lens system, and more particularly to a high-magnification zoom lens system used with compact cameras.
In the development of zoom lenses for compact cameras, there is growing demand for achieving a much higher zoom ratio equal to that of an interchangeable lens for single-lens reflex cameras. As already proposed by the inventors, a high-magnification zoom lens system which comprises three lens groups, positive, positive and negative, and is a development of a four-group zoom type having a positive-negative-positive-negative power profile in order from an object side thereof can achieve a zoom ratio exceeding 4. In this zoom lens system, the final lens group having negative refracting power takes part in the zooming action. For zooming, the final lens group moves in association with other lens groups. However, this zoom lens system is practically less acceptable because some problems still remain unsolved, e.g., a problem regarding the amounts of movement of lens groups during zooming, a decentration error problem, and a sensitivity problem during focusing. In JP-A 4-303809, the inventors have proposed to provide an extension of a wide-angle side by varying spaces between lens groups at a wide-angle end, thereby achieving an ever higher magnification. In JP-A 3-31809, the inventors have also proposed to provide an extension of a telephoto side by varying spaces between lens groups at a telephoto end.
However, these inventions have for their object to extend a field angle with respect to a zoom lens, and so have no direct relation to a zoom lens having a high zoom ratio or an optical system involving an extending method.